1. Field of the Invention
The present invention relates in general to a hydraulic cushioning apparatus for a press, which has hydraulic cylinders linked with pressure pins, and more particularly, to such a cushioning apparatus which is capable of applying a cushioning force to a workpiece uniformly through the pressure pins, over a wide range of the cushioning force.
2. Discussion of the Prior Art
A drawing press, for example, is equipped with a hydraulic cushioning apparatus, which includes a pressure pad or ring that is operated by a plurality of pressure pins, to force a portion of the workpiece against a die or punch, for preventing wrinkling of the workpiece and assuring high surface quality of the formed or drawn piece. While the required cushioning force is transmitted to the pressure pad through the pressure pins, the force or load acting on one pressure pin may differ from that acting on the other pressure pins, due to a slight difference in the length of the pins, variations or errors in the relative positions of the other components (e.g., cushion platen) of the cushioning apparatus, and wearing of the components. For instance, the different lengths of the pressure pins cause different contacting pressures of the pins with respect to the pressure pad, and/or a spacing between the ends of some of the pins and the facing surface of the pressure pad, which spacing results in the failure of those pins to transmit any cushioning force. Thus, the cushioning force may be unevenly distributed to the pressure pins.
To avoid such uneven distribution of the cushioning force to the pressure pins, the pressure pins are linked, at their ends remote from the pressure pad, to the pistons of respective hydraulic cylinders, as disclosed in laid-open Publications Nos. 1-60721 and 2-39622 of unexamined Japanese Utility Model Applications. The hydraulic cylinders function to absorb the dimensional and/or positional variations or errors associated with the pressure pins indicated above, so that substantially the same cushioning force is transmitted through each of the pressure pins, so as to assure uniform cushioning pressure acting on the surface of the pressure pad over the entire working surface.
It is necessary to consider the conditions in which all the pressure pins are correctly operable to transmit substantially the same cushioning force to the pressure pad, for uniform cushioning pressure on the pressure pad. Generally, an average operating stroke Xav of the hydraulic cylinders (pressure pins) is represented by the following equation (1): EQU Xav=(F-nSP.sub.0)V.sub.0 /(n.sup.2 S.sup.2 K) (1)
where,
P.sub.0 : initial hydraulic pressure to be applied to the hydraulic cylinders; PA1 F: required cushioning force F to be applied to the pressure pad; PA1 S: cross sectional area of the piston of each hydraulic cylinder; PA1 n: number of the hydraulic cylinders (pins); PA1 K: volume modulus of elasticity of the working fluid
According to the above equation (1), a relationship among the cushioning force F, number n of the pressure pins and average operating stroke Xav of the hydraulic cylinders is represented by a graph as shown in FIG. 9, in which the cushioning force F is taken along the horizontal axis while the number n of the pressure pins is taken along the vertical axis.
If the average operating stroke Xav of the hydraulic cylinders is too small, the relatively short pressure pins may not function to transmit the cushioning force, due to the spacing between the upper ends of those short pressure pins and the pressure pad. If the average operating stroke Xav is too large, on the other hand, some of the pressure pins may be bottomed with their lower ends reaching the lower stroke end, namely, the pistons of the corresponding hydraulic cylinders are bottomed, when the speed of downward movement of the upper movable die (press slide) is too high at the time when the movable die collides with the workpiece to force the workpiece against the pressure pad. Thus, the cushioning force cannot be evenly distributed to the pressure pins, or the pressure pad cannot be uniformly pressed against the workpiece by the pressure pins, if the average operating stroke of the hydraulic cylinders (pressure pins) is too large or too small.
For the above reason, the average operating stroke Xav should be held within an optimum range R between a certain lower limit and a certain upper limit, for example, between Xb(mm) and Xd(mm), as indicated by a hatched area in FIG. 9.
It will be understood from the above equation (1) that the optimum range R changes with the initial hydraulic pressure P.sub.0 to be applied to the hydraulic cylinders, a total amount V.sub.0 of the fluid in the hydraulic cylinders, the cross sectional area S of each hydraulic cylinder, and the volume modulus K of the fluid.
However, the known hydraulic cushioning apparatus is not capable of changing the initial hydraulic pressure P.sub.0. Further, the fluid volume V.sub.0 and cross sectional area S of the hydraulic cylinders, and the volume modulus K of elasticity of the fluid are fixed. Therefore, the optimum range R is fixed, and cannot be changed. Usually, the number n of the pressure pins is fixed, but the required cushioning force F is changed to meet the particular material and thickness of the workpiece, or changed in steps for the purpose of finding out the optimum pressing condition, in a test pressing operation. Accordingly, the initially selected cushioning force F which falls within the optimum range R may be changed to a value outside the optimum range R.
To perform pressing operations with the suitable cushioning force F within the optimum range R, the number n of the pressure pins or the structure of the die assembly of the press should be changed. This requires considerable labor and time, and is not practically possible.
Moreover, the uneven distribution of the cushioning force F to the pressure pins, or the bottoming of the cylinder pistons, cannot be detected until a pressing operation on the workpiece is finished. Namely, those defects of the cushioning apparatus can be detected only after the finding of the formed or drawn pieces having poor quality due to the defects of the cushioning apparatus.